how to add dll to project?

how do i add a dll to my project ? i tried adding it to the assets folder as suggested in several other posts, but when i try to use the "using dllname; it simply doesn’t appear, if i try to add the reference manually i get assembly errors in unity.

the DLL is System.Data.SQLite with target framework being .net 3.5

dll to game its dont work and its uselless